Airplane manufacturers are under increasing pressure to produce lightweight, strong, and durable aircraft at the lowest cost for manufacture and life cycle maintenance. An airplane must have sufficient structural strength to withstand stresses during flight, while being as light as possible to maximize the performance of the airplane. For these reasons and others, aircraft manufacturers have increasingly used fiber-reinforced resin matrix composites.
Fiber-reinforced resin matrix composites provide improved strength, fatigue resistance, stiffness, and strength-to-weight ratio by incorporating strong, stiff, carbon fibers into a softer, more ductile resin matrix. The resin matrix material transmits forces to the fibers and provides ductility and toughness, while the fibers carry most of the applied force. Unidirectional continuous fibers can produce anisotropic properties, while woven fabrics produce quasi-isotropic properties.
In one prior art method of producing fiber-reinforced resin matrix components for aircraft, a number of prepreg sheets were stacked on a lay-up mandrel. An example of such a process is disclosed in U.S. Pat. No. 4,765,942, incorporated herein by reference. The lay-up mandrel included internal plumbing extending to a number of vacuum ports at the upper surface of the lay-up mandrel. The vacuum ports were located on a circumference that extended a few inches outside the perimeter of the prepreg sheets located on the lay-up mandrel.
A parting film, such as fluorinated ethylene propylene (FEP), was applied over the stack of prepreg sheets. A flexible, fiberglass breathable blanket was placed over the top of the parting film. The breathable blanket was larger than the prepreg sheets so that it contacted the vacuum ports at the surface of the lay-up mandrel. A vacuum bag was placed over and sealed around the entire structure, and the lay-up mandrel was placed in an autoclave and the prepreg sheets were cured.
During curing, vacuum was applied to the bag and the autoclave was pressurized so as to compact the prepreg sheets onto the upper surface of the lay-up mandrel. After the curing process was complete, the compacted and cured composite part was removed from the lay-up mandrel.
The curing process described above was not successful in formation of high quality, large thermoplastic composite parts. The prepreg sheets used in formation of thermoplastic composite parts were "wetted," or soaked with solvents, to carry the resin onto the fibers and create a tack similar to conventional thermoset composite prepreg sheets. The introduction of solvents into the prepreg sheets required that the solvent, as well as a number of other by-products of the curing process, such as ethanol, water, and volatiles (typically 15% by weight), be removed before and during the curing process to form a high quality composite part. The by-products may evolve during the heating or curing cycles, can be a residual solvent carried in the material, or can be produced as a result of the condensation and curing reactions that occur at elevated temperatures to convert the resin to a composite. The by-products can include water, solvents, and volatiles that are emitted by the resin during curing of composite material. The volatiles include organics that are the residue of protecting groups on the capping compounds of the monomer reactants of the resin, and organics that are emitted from the capping compounds when the capping compounds release their protective groups and react.
In prior art processes, the removal of by-products of the curing process was facilitated via the vacuum ports distributed around the perimeter of the lay-up mandrel. The breathable blanket provided a flow path for the by-products during vacuum.
Past experience has demonstrated that high-quality small thermoplastic parts can be fabricated using the composite forming methods described above. However, in large parts, where the flow path from portions of the composite part to the vacuum ports was greater than 1.5 feet, pores (pockets where no resin is present around the fibers) were produced at portions of the part removed from the vacuum parts as a result of air, moisture, and other by-products not being drawn during the curing process. The pores caused a drop in strength of the composite part.
To remedy this problem, prior processes increased the number of vacuum ports around the part. The results were mixed, and the majority of prior art processes were not successful in the fabrication of parts that have a flow path to the vacuum ports from central portions of the part that was greater than 1.5 feet (i.e., a part that was greater than 3 feet across).
The prior art used staged heating to drive off the excess solvent prior to reaching the melt and cure temperatures where the capping compounds released their protecting groups and then reacted. Other prior art processes increased the time to draw the by-products off, both during the heat-up and the dwell of the cure cycle. It was assumed that given enough time, a sufficient amount of the by-products would be removed from the part. One process lengthened the dwell time from 2 hours to 8 hours. This modification in the cure cycle resulted in an increase of maximum by-product flow path from 1.5 feet to 3 feet. However, the increase in dwell time was insufficient for creating composite parts that were greater than 6 feet across. This limitation made it impossible to make large panels like wing skins.
Another attempt to cure large thermoplastic panels involved the use of a lay-up mandrel made of a porous material such as monolithic graphite. The porous material permitted the extraction of some by-products directly through the lay-up mandrel, or along the lay-up mandrel's surface. One problem with this approach was that the monolithic graphite had poor strength, resulting in the need for structural support for rigidity and handling. The combined weight produced an extremely heavy lay-up mandrel, which was difficult to handle in a factory. The monolithic graphite was also difficult to machine. In addition, monolithic graphite chipped and cracked easily, making it an impractical material for a production environment.
Thus, there exists a need for an improved method for making a large fiber-reinforced resin matrix composite part substantially free of pores.